How to texture specific faces/objects
Seems like a basic question but I'll give it a go.

Is the classroom one big object? In the top-right corner, there is a hierarchy. It currently reads "Lamp, Lamp001, Lamp006..." Is the geometry in this scene represented by one entry? If not, go to the next paragraph. If so, it should first be split into multiple objects. Each desk, table, etc. should be its own object. To split up the mesh one object at a time: select the faces belonging to an object, press p (no ctrl or shift) to bring up the separate menu, then choose "selection." It should now be its own object. You will have to repeat this for every individual object.

Some objects will need multiple textures. For example, a desk might have a plastic back but metal legs. To do this, go to edit mode and look at the materials tab. There should be an "assign" button. Clicking it will assign the currently selected material in the materials tab (there can be multiple, click the plus sign on the side to add another) to all selected faces in the viewport or 3d view. This way, an object can have multiple materials each with its own texture.